Almost half of the courtrooms handling the most serious criminal cases in London will be closed down during the Olympics, according to official plans.

Her Majesty's Court and Tribunal Service (HMCTS) said the reduced service was necessary due to fears that transport delays caused by the Games could prevent victims, jurors and lawyers travelling to and from courts.

Lawyers warned last night the Government must be prepared to step in to ensure the wheels of justice continue to turn smoothly. Courts such as the Old Bailey, which handle some of the most serious crimes, will be affected as will some magistrates courts, which deal with high volumes of suspects on a daily basis.

Last August's riots saw London's Magistrates so stretched that some had to sit through the night to deal with the numbers arrested. A spokesman for the Bar Council – which represents barristers in England and Wales – said he felt sure the Government would have a "contingency plan" in place in case of an emergency.

"Whilst adjustments to some public services will inevitably be made during the Olympic Games, it is essential the justice system continues to operate smoothly and efficiently, in courts around the capital, as a priority," he said. "As ever, the Bar will play its role in ensuring the public can have confidence in the handling of criminal cases. The Government must ensure that it plays its part too."

The plans, by the HMCTS Olympic Planning Group, would see Crown courts near key venues and travel hotspots stop sitting altogether or offer only a reduced service during the 17 days of the Games, putting pressure on some courts to finish trials early.

A spokesman for HMCTS said: "Most of the courts and tribunals in London will be operating normally during the Olympics, and will deal with all urgent applications and cases. We have only reduced court and tribunal sittings at those buildings close to Olympic venues and known travel hotspots, and are in the process of rescheduling listings to ensure that any displaced work is dealt with either before or in the weeks immediately after the Games.

"The HMCTS Olympics Planning Group is still in the process of considering and agreeing with partner agencies what contingency plans need to be put in place in the event that there is a higher level of crime than expected."

Of London's 138 courtrooms across 11 Crown courts, only between 72 and 75 will be sitting. The Royal Courts of Justice will hear only emergency applications on 30 and 31 July.

The affected courts include Southwark, near the London Bridge transport hotspot; Snaresbrook, which is three miles from the Olympic Park in east London; Woolwich, which is nine miles away; and the Old Bailey in central London.

At the Royal Courts of Justice "jurisdictions will aim to complete trials during the week July 23-27" and the normal summer recess from 1 August "will reduce hearings to a sustainable level".

Magistrates' courts across the capital will also see significant changes. Stratford will use just one courtroom, dealing with overnight cases, and planned youth courts will not be held there. Thames magistrates will also only use one courtroom and Westminster will use five.